A position encoder system is described in copending U.S. patent application Ser. No. 07/856,436, entitled "Position Encoder System," which is herein incorporated by reference. The position encoder system described in that application is embodied as an input device for a "notebook" type computer. The input device includes a small handle that is manipulated by the user to control, for example, the position of a cursor on the computer screen. The handle includes two push-button type switches that can be depressed to activate any of a number of functions, such as pulling down a computer program menu.
The handle is carried on the end of a somewhat rigid, elongated link. The link is connected to a carrier that is inside of the computer. The link slides in the carrier, and the carrier rotates, thereby allowing motion of the link in two dimensions. The link's sliding motion and the carrier's rotation are measured to provide data to track the handle's position, which can be correlated to the cursor position.
The preferred technique for measuring the link sliding motion and the rotational motion of the carrier includes a pattern of conductors printed on one side of the link to move with the link relative to an adjacent conductive pattern that is mounted to the carrier. The pattern on the carrier is driven with a series of moving wave signals that are capacitively coupled to the pattern on the link, those signals being returned (again, via capacitive coupling) to the carrier. The returned signals are phase modulated as a result of movement of the link relative to the carrier. The phase-modulated signals are detected and processed to yield digital information indicative of the motion of the link. No direct conductive path between the link and the computer is necessary to provide the motion information.
In the above-mentioned position encoder system, information representing the state of the switches on the handle is conveyed to the computer through conductive paths. The conductive paths may take the form of either wires or sliding contacts connected between the link and carrier. Both forms of conductive paths, however, pose problems. If wires are used to connect the link to the computer, a considerable volume of interior space will be consumed to house the slack wires needed to permit movement of the link in its carrier inside the computer. Also, the wires must be very flexible and arranged to avoid interference with the free motion of the handle.
Sliding contacts also require a significant amount of the interior space of the computer and may interfere with the free movement of the handle due to the friction necessarily applied by the contacts to the sliding link. Such sliding contacts are subject to considerable wear. A reliable sliding contact is difficult to manufacture and maintain because dirt and other foreign matter may enter the contact area.